On the origin of the high strain rate sensitivity in a superplastic Al-4.5% Mg alloy

Abstract

During high temperature plastic deformation, strain rate sensitivity of flow stress is a very important parameter. If cavitation can be prevented by applying a back pressure when forming a component, and work hardening is negligible, high values of strain rate sensitivity index (usually denoted as m) then essentially determine the ductility of a material. When m = 1 and the strain rate is a linear function of the applied stress, a material might be deformed indefinitely just like fluid glass and the process is known as Newtonian viscous flow. Having high m is the most important feature of superplastic materials and its origin or the cause has been the focus of research but not well identified. Newtonian viscous flow may be achieved through various mechanisms during plastic deformation in crystals, among which diffusional flow of atoms is well established both theoretically and experimentally. Dislocation creep can exhibit Newtonian viscous flow when the mobile dislocation density is a constant and independent on the flow stress during the deformation. Grain boundary sliding (GBS) is very widely considered to be significantly involved during structural or fine grained superplastic deformation. Many models based on this have suggested m = 0.5 except that by Ashbymore » and Verrall and that by Spingarn and Nix. The nature of superplasticity is in fact very complex and it appears that some of the most important features involved such as the activation energy and the maximum magnitude of m as a function of strain rate and temperature vary with materials. It is proposed that Newtonian viscous flow may be achieved by lattice dislocation controlled GBS and the superplasticity is the combination of this process and conventional dislocation creep in so-called class 1 solid solutions. It is the intention of this paper to note a possible similarity between the two kinds of superplasticity in terms of the origin of the high strain rate sensitivity.« less